Thermal transfer printing is one type of non-impact printing which is becoming increasingly popular as a technique for producing high quality printed materials. Applications exist in providing low volume printing such as that used in computer terminals and typewriters. In this type of printing, ink is printed on the face of a receiving material (such as paper) whenever a fusible ink layer is brought into contact with the receiving surface, and is softened by a source of thermal energy. The thermal energy causes the ink to locally melt and transfer to the receiving surface.
The thermal energy is supplied from either an electrical source or an optical source, such as a laser. When electrical sources are used, a thermal head can provide the heat to melt the ink layer. An example of a thermal head is one which consists of tantalum nitride thin film resistor elements, as described in Tokunaga, et al, IEEE Trans. on Electron Devices, Vol. ED-27, No. 1, January 1980, at page 218.
Laser printing is known in which light from laser arrays is used to provide the heat for melting and transferring the ink to a receiving medium. However, this type of printing is not very popular because lasers providing sufficient power are very expensive.
Another type of thermal transfer printing is one in which a resistive ribbon is provided containing a layer of fusible ink that is brought into contact with the receiving surface. The ribbon also includes a layer of resistive material which is brought into contact with an electrical power supply and selectively contacted by a thin printing stylus at those points opposite the receiving surface that are desired to be printed. When current is applied, it travels through the resistive layer and provides local resistive heating in order to melt a small volume of the fusible ink layer. This type of printing is exemplified by U.S. Pat. No. 3,744,611. An electrothermal printhead for use in combination with a resistive ribbon is shown in IBM Technical Disclosure Bulletin, Vol. 23, No. 9, February 1981, at page 4305.
In resistive ribbon thermal transfer printing, it is often the situation that the substrate contact to the head becomes unduly heated and debris accumulate on the printhead. This increases the contact resistance and develops heat in the printhead. To overcome the accumulation of debris and the increase in contact resistance, the amplitude of the applied current has to be increased. This can produce fumes and ruin the substrate.
A technique for reducing the amount of power required within a printhead in a resistive ribbon thermal transfer process is described in IBM Technical Disclosure Bulletin, Vol. 23, No. 9, February 1981, at page 4302. In this approach, a bias current is provided through a roller into the resistive layer located in the printing ribbon. This means that not all of the energy required to melt the ink has to be supplied through the printhead.
Another approach possibly providing some amplification of heat is that described in IBM Technical Disclosure Bulletin, Vol. 20, No. 2, July 1977, at page 808. In this reference, a photoconductive layer is located between two electrodes, across which is attached a power supply. When light strikes the photoconductor, it will be conductive in the region where it is hit by the light, and will close the circuit between the two electrodes. This provides a current flow where the current is a source of heat that develops in the photoconductor and is transferred to an adjacent ink layer. The ink layer is locally melted so that it can be transferred to a receiving medium.
In thermal transfer printing, it is known that the ink transfer efficiency and print quality depends upon the pressure, the thickness of the ink layer and the base, and the smoothness of the ink layer on the paper surfaces. These factors affect transfer efficiency and print quality for the same heating power and heat duration.
In the present invention, a technique has been discovered for alleviating some of the power requirements in thermal transfer printing. This technique is available to printers in which a thermal head (including laser print heads) is used to provide heat, and to printers in which resistive ribbons are used. Rather than using mechanical or electrical techniques for reducing the amount of power that is required to print, the present invention chemically provides heat amplification in any type of thermal transfer printing. This is accomplished by using an exothermic material which undergoes an exothermic reaction and is located close to, or in the ink layer. Application of a heat pulse or a current pulse in a printhead is merely a trigger to cause the exothermic material to locally produce heat, which aids in melting and/or transferring the ink.
Accordingly, this invention addresses some of the problems present in all types of thermal transfer printing, and has for a primary object a reduction in the amount of power required for thermal transfer printing.
It is another object of the present invention to provide thermal transfer printing of any kind in which clearer print images are provided with the same input power as would be used in printing applications without the improvement provided by the present invention.
It is another object of this invention to provide improved thermal transfer printing in which debris which accumulates in the printhead is reduced by reducing the magnitude of the required print current.
It is a further object of this invention to provide improved thermal transfer printing in which the contact time between the printhead or print stylus and the ink containing ribbon or layer is reduced.
It is another object of this invention to provide a technique for prolonging printhead life and for reducing the presence of fumes in thermal transfer printing.
It is another object of this invention to provide an inexpensive way to reduce power requirements in all types of thermal transfer printing.
It is another object of this invention to improve laser printing techniques in a manner to make them more economically feasible.
It is a further object of this invention to reduce current power requirements in thermal transfer printing in a manner which does not produce adverse or toxic fumes.